Chatter and vibration are problems in the high-speed machining of metals with rotary cutting tools. Indeed, the limiting factor for metal removal rates in high-speed machining of metals is the dynamic stability of the tool. For a given rotary cutting tool such as an endmill or drill bit, there exists a set of rotational speed and depth of cut combinations at which the tool will become unstable and chatter or vibrate with sufficient amplitude that the quality of the machined surface is seriously compromised and/or the tool becomes damaged. It will be appreciated that, in order to improve efficiency of a cutting operation, it is desirable to be able to delay the onset of cutter instability to as high a rotational speed as possible so that higher metal removal rates can be accomplished.
Cutter stability can be achieved at higher rotational speeds by management of the dynamics of the cutter system which typically includes the cutting tool, tool holder, and spindle of the machine. At lower rotational speeds, a phenomenon known as "process damping" can provide cutter stability. Process damping is a phenomenon that occurs during machining with a rotary cutting tool, in which the radial relief surfaces of the tool come into contact with and rub against the workpiece being machined as a result of waviness left on the machined surface caused by cutter vibration. This rubbing action damps the cutter vibration, thereby reducing or eliminating the vibration. For a given tool, process damping will occur whenever the rotational speed is sufficiently low to allow the relief surfaces to rub against the workpiece.
For cutting tools having a large length-to-diameter ratio, the problem of chatter is particularly vexing because such tools are inherently less capable of taking advantage of system dynamics than tools having smaller length-to-diameter ratios, since the tool represents the most flexible part of the system. Thus, for all rotary cutting tools, and particularly for those relatively flexible tools having larger length-to-diameter ratios, it is beneficial in terms of part production rates and production efficiency to increase the limiting speed at which dynamic stability can be achieved through process damping.
A number of approaches have been tried for improving the dynamic stability of rotary cutting tools by increasing the damping of the cutting system of which the tool forms a part. Typically, damping is increased by adding what essentially amounts to a spring-mass damper to the cutting tool. For example, U.S. Pat. No. 2,426,359 discloses a boring bar in which an insert of a damping material is provided in the shank portion of the boring bar to increase the overall damping of the tool. U.S. Pat. No. 3,292,237 discloses a cutting tool known in the art as an insert tool in which an insert having a cutting edge is made of a very hard material and is affixed in a body portion of a tool. The patent discloses coating the body portion of the tool with a plastic coating in order to increase the overall damping of the tool.
Similarly, U.S. Pat. No. 5,033,579 discloses a circular saw blade in which a coating of a damping material is applied to the circular side surfaces of the blade in order to increase the overall damping of the blade.